To adapt to the rolling resistance and rolling speed at the time, manual transmissions in some types of motor vehicles, particularly heavy utility vehicles, have more than one starting gear. Accordingly, when starting up from a coasting vehicle situation, the driver is faced with the problem of selecting and engaging the starting gear best suited for the current rolling resistance, determined essentially by the load status and the road inclination, and the current rolling speed.
In this connection the term ‘coasting vehicle’ is understood to mean a forward movement of the motor vehicle within a speed range above a lower reference speed close to the vehicle's being at rest and below a higher reference speed corresponding to a low driving speed, during which the transmission is in the neutral driving position N and/or the clutch is completely disengaged. Coasting of the vehicle can occur during ordinary driving operation, for example in urban traffic in the form of coasting before reaching a red traffic-light, from which, in the chosen example when the light turns green, it is often desired to accelerate again without the motor vehicle previously having come to rest. But vehicle coasting can also take place from rest, if the motor vehicle is on a downhill stretch and begins rolling when the parking brake or the working brake is released but before a starting gear has already been engaged and/or the clutch engaged.
It is true that with an automated drivetrain of the type mentioned at the beginning, on starting up from the coasting condition in most cases the starting gear to be engaged is determined automatically at least as a function of the current driving or rolling speed and perhaps also of the current deflection path of the accelerator pedal. However, unless at the same time the current load status and/or the road inclination are also determined sufficiently accurately and taken into account when determining the starting gear, the starting gear determined automatically may not be suitable for the actual driving situation at the time. In such a case, in a modern automated drivetrain the possibility exists that the starting gear determination can be influenced by the driver by actuating the manual shift lever, or that the starting gear determination carried out automatically by the transmission control unit can be corrected or changed by the driver by actuating the manual shift lever.
A first drivetrain of this type is described in EP 0 602 685 B1. This known drivetrain also comprises a manual shift lever which, in the upshift direction and in the downshift direction has a respective first or third shift position close to the neutral position and a second or fourth shift position farther away from the neutral position, and which is also provided with at least one pressure switch.
With the drivetrain, if the driving speed is above a specified minimum speed, a movement of the manual shift lever to the first shift position when a gear is engaged brings about a shift to the next-higher gear, provided that in the next-higher gear the rotation speed limits of the drive motor are respected; otherwise, the shift command is ignored. When the transmission is in its neutral position or if at the same time the pressure switch is actuated, then in contrast movement of the manual shift lever to the first shift position initiates a shift to the highest permissible gear. If the measured shift lever is moved to the second shift position, then if the speed is higher than the specified minimum value and a gear is engaged, then provided this is permissible a shift takes place to the second-higher gear or, if necessary, to the next-higher gear. In contrast, if the transmission is in neutral or if the pressure switch is actuated at the same time, movement of the manual shift lever to the second shift position also brings about a shift to the highest permissible gear.
If the manual shift lever is moved to the third shift position, then if the speed is higher than the specified minimum and a gear is engaged, provided that this is acceptable, a shift to the next-lower gear takes place. In contrast, if the transmission is in neutral or if at the same time the pressure switch is actuated, movement of the manual shift lever to the third shift position beings about a shift to the lowest permissible gear. If the manual shift lever is moved to the fourth shift position, then if the speed is higher than the specified minimum speed and a gear is engaged, provided that this is permissible, a shift takes place to the second-lower gear or if necessary to the next-lower gear. If the transmission is in neutral or if at the same time the pressure switch is actuated, then movement of the manual shift lever to the fourth shift position initiates an emergency shift to a still permissible, extreme low gear.
Thus, the known drivetrain offers a number of operating options which, however, can also lead to erroneous operations by the driver. Besides, no distinction is made between vehicle coasting at a relatively low travel speed and an open drivetrain at higher speed. Disadvantageously, the driver is assisted only indirectly when selecting an optimum starting gear, namely in that the engagement of an impermissible gear is prevented.
Another drivetrain of this type is known from U.S. Pat. No. 5,664,458 A. In this drivetrain a manual shift lever is provided, which can be moved in an upshift direction for the manual initiation of an upshift and in a downshift direction for the manual initiation of a downshift. When the transmission is shifted to its neutral position and the current driving speed is above a specified minimum reference speed, a “start from vehicle coasting” operating mode is activated, in which a single movement of the manual shift lever in the upshift direction brings about the engagement of an automatically determined gear appropriate for the travel speed at the time.
If the drivetrain comprises a clutch that can be actuated by the driver by means of a clutch pedal, for this function a signal from an actuation sensor that indicates the disengaged condition of the clutch must also be present. Thus, in selecting the appropriate starting gear the driver is assisted directly, but has to actuate the manual shift lever for this. Since the starting gear is automatically determined only as a function of the current travel speed, this preselected starting gear may not be appropriate and this may sometimes call for corrective action by the driver in the form of further manual shift lever operations. In the case of this drivetrain too, no distinction is made between a vehicle coasting at relatively low speed and a disengaged drivetrain at higher speed.